The rapid advancement of multimedia applications require a system that stores more information more compactly, and is easier to use and handle. Thus, digital video disc (DVD) technology offers this high density format advantage. DVD technology is based on a new generation of a compact disc format which provides for increased storage capacity and performance, especially for video and multimedia applications.
Compact disc (CD) technology is aggressively becoming the global standard for music, entertainment and computer software. Although extremely popular, CD technology has yet been able to provide high density formats. A compact disc (CD) can typically only store approximately five minutes worth of analog image information, and a larger size laser disc of 11.8 inches can store approximately sixty minutes of analog image information. Accordingly, while the movie/entertainment and computer industries want to pursue high density formats, such as that found in DVD technology, of concern is the maintenance of CD compatibility due to its appearance throughout the industry.
As the industry introduces this new digital technology, an increasingly important consideration is compatibility with the previous formats. Great effort has gone into making the DVD system, such as DVD-ROM, backward compatible. Backward compatibility includes the ability to play the existing CD format on a DVD system, while keeping in mind that there exist quite a few physical differences between a DVD system and a CD system. For instance, a DVD format disc has a pit length of 0.4 .mu.m and a track pitch of 0.74 .mu.m, almost half that of a CD format disc. In addition, the typical DVD substrate is 0.6 mm, in comparison to a typical CD substrate of 1.2 mm. The DVD laser wavelength is either 635 nm or 650 nm, and the DVD numerical aperture (NA) of the focal lens is 0.6, while the CD laser wavelength is typically 780 nm, and the CD NA of the focal lens is 0.45. These different parameters between DVDs and CDs lead to different laser requirements for read/write applications.
Several approaches have been proposed to solve the DVD backward compatibility issue. Among the solutions are an integrated two-lens switching system which consists of two lenses, one for CD, and the other for DVD, and a dual focus pickup system in which a single lens is used to achieve focus for both CD and DVD systems. With the integrated two-lens switching system, two lenses, one for the CD and the other for the DVD, are switched by rotating the lenses horizontally to read signals for each disc. With the dual-focus pickup system, a single lens can achieve focus for both the CD and the DVD without being realigned, and read the signals, due to the adoption of a hologram lens.
The two-lens system and the dual-focus lens system are complicated and difficult to make. Another technique is to use an LCD shutter to alter the characteristics of the laser used in reading the discs. The LCD shutter pickup is less complicated than the two-lens and dual-focus formats, resulting in a smaller, more reliable pickup. In this instance, the pickup is built using a 635 nm red laser. The LCD shutter changes the numerical aperture of the lens, producing a narrower beam, resulting in a longer focal distance of 1.2 mm into CDs, or a wider beam, resulting in a shorter focal distance of 0.6 mm into DVDs. This allows reading the two different formats with one pickup.
Although the LCD shutter pickup is an improvement to either the two-lens pickup system or the dual-focus lens pickup system, it is not an efficient use of the laser output power because the shutter blocks a significant amount of the laser power to narrow the beam width. In many instances, power efficiency is very important, such as in laptop computer applications. In addition the LCD shutter pickup requires extra circuitry to control the LCD shutter.
In general, longer wavelength lasers, such as 780 nm lasers, are widely used in low density compact disc (CD) read and write applications. This next generation of high density DVDs will require shorter wavelength lasers, such as 650 m or lower for read and write applications.
As previously described, vertical cavity surface emitting lasers (VCSELs) can potentially be utilized for these read and write applications and are capable of operating at a specific wavelength dependent upon the material system utilized in the fabrication process. In general, VCSELs include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL is driven by current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack.
It would be highly advantageous to remedy the foregoing backward compatibility challenge and the deficiencies inherent in the prior art. Thus it is highly desirable and an object of the present invention to provide for a reading head, or optical pickup module, that incorporates both a short and a longer wavelength device. Accordingly, proposed is a low cost semiconductor chip that includes dual wavelength monolithically integrated VCSELs of different wavelengths, thereby capable of being utilized to read and write information to and from both CDs and DVDs, thus fulfilling the requirements of backward compatibility.
Another object of the invention is to provide a reliable semiconductor laser chip that incorporates a longer wavelength VCSEL, more particularly one emitting light of approximately 780 nm and a short wavelength VCSEL, more particularly one emitting light of approximately 635 to 650 nm, thereby capable of being utilized with both CDs and DVDs.
It is yet another object of the present invention to provide for efficient active regions and mirror stacks for use in a semiconductor chip that includes dual wavelength monolithically integrated VCSELs.
Still another object of the present invention is to provide for a method of fabricating dual wavelength monolithically integrated vertical cavity surface emitting lasers that includes both a longer wavelength VCSEL and a shorter wavelength VCSEL for use with both CDs and DVDs.
Yet another object of the invention is to reduce the complexity, thus cost, of monolithically integrating a long wavelength VCSEL and a shorter wavelength VCSEL.